Modifying starch with cationic polymers and use of the modified starches as dry-strength agent

ABSTRACT

A process for modifying starch by heating starch in an aqueous medium to temperatures above the gelatinization temperature of said starch comprises effecting said modifying of said starch in the presence of 
     (a) a polymeric cationizer selected from the group consisting of the polymers containing vinylamine units and having molar masses of up to 1 million, the polyethyleneimines, the polydiallyldimethylammonium chlorides, the condensates of dimethylamine with epichlorohydrin or dichloroalkanes, the condensates of dichloroethane and ammonia, and the mixtures thereof, and 
     (b) polymeric papermaking drainage aids selected from the group consisting of the water-soluble crosslinked polyamidoamines with or without an ethyleneimine graft, the polymers containing acrylamide and/or methacrylamide units and having molar masses of more than 1 million, the polymers containing vinylamine units and having molar masses of more then 1 million, and the mixtures thereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for modifying starch byheating starch with at least one cationic polymer in an aqueous mediumto temperatures above the gelatinization temperature of said starch, tothe modified starches obtainable by said process, and to the use of saidmodified starches as paper, paperboard and cardboard dry strengthenhancer.

2. Description of the Background

Ullmann's Enzyklopädie der Technischen Chemie, 4^(th) edition, VerlagChemie, Weinheim, 1979, vol. 17, pages 581 ff., gives a generaldescription of the use of digested, water-soluble starches and ofwater-soluble starch derivatives as assistants for papermaking, inparticular for enhancing the strength of paper. However, the retentionof the dissolved starches in paper is low, leading to a considerablewastewater loading.

U.S. Pat. No. 3,467,608 describes a process for preparing a cationicstarch by heating an aqueous starch slurry in the presence of apolyalkyleneimine, for example polyethyleneimine, or of apolyalkylenepolyamine, for example polyethylenepolyamine. The molecularweight of the modifying polymer used is not less than 50,000 in eithercase. The reaction mixture contains 0.5 to 40% by weight of the polymerand 60 to 99.5% by weight of starch, the percentages being based on thesolids content. The possibility of using starches modified byhydrolysis, oxidation, esterification and etherification is describedhere in general terms. The resulting cationic starch derivatives areused as flocculants.

U.S. Pat. No. 4,097,427 describes a process for cationizing starch byreacting an aqueous starch slurry under alkaline reaction conditions inthe presence of an oxidizing agent with a water-soluble polymer whichcontains quaternary ammonium groups. Only unmodified or minimallydegraded starches are used, preferably maize and tapioca starch. Thepolymers containing quaternary ammonium groups are selected frompolymers containing epichlorohydrin units, quaternizedpolyethyleneimines, quaternized diallyldialkylamine polymers, etc. Theoxidizing agent is for example ammonium persulfate, hydrogen peroxide,sodium hypochlorite, ozone or tert-butyl hydroperoxide. The pH of thestarch slurry is adjusted to about 8 using water-soluble bases. Themodified cationic starches thus prepared are added to paper stock toenhance the dry strength of the paper being produced. In this process,however, the wastewater has a very high chemical oxygen demand (COD).

U.S. Pat. No. 4,146,515 discloses a process for preparing cationicstarch used for surface sizing and coating paper and paper products. Inthis process, an aqueous slurry of oxidized starch is digested togetherwith a cationic polymer in a continuous cooker. The cationic polymersare selected from the group consisting of epichlbrohydrin-dimethylaminecondensate, diallyldimethylanmonium chloride polymer, quaternizedethylene chloride dichloride ammonia reaction product and alsoquaternized polyethyleneimine, inter alia.

DE-A-3719480 discloses a process for producing paper, paperboard andcardboard by using a dry strength enhancer comprising a mixture obtainedby heating native potato starch in an aqueous medium in the presence ofpolyethyleneimine at above or below the gelatinization temperature ofthe starch.

EP-A-0282761 discloses a process for producing paper, paperboard andcardboard by using a dry strength enhancer comprising a mixtureobtainable by heating native potato starch with polyvinylamine inaqueous solution at 70 to 110° C.

EP-A-0301372 discloses a process for producing paper of high drystrength by using a dry strength enhancer comprising a mixture obtainedby heating enzymatically degraded starch and cationic polymers such aspolyvinylamines at up to 170° C.

U.S. Pat. No. 4,880,497 and U.S. Pat. No. 4,978,427 disclose theproduction of paper of high dry and wet strength by adding, either tothe surface of the paper or to the paper stock prior to sheet formation,a hydrolyzed copolymer strength enhancer obtainable by polymerizingN-vinylformamide and ethylenically unsaturated monomers such as vinylacetate or alkyl vinyl ethers and hydrolyzing from 30 to 100 mol % ofthe formyl groups of the copolymer to form the amino groups.WO-A-96/135525 [sic] discloses a process for the cationic modificationof starch by reacting starch with polymers, diamino [sic] and/orammonium groups contain [sic] in an aqueous medium at 115 to 180° C.under superatmospheric pressure with only minimal degradation of thestarch.

WO-A-98/31711 discloses a process for modifying starch by, for example,reacting an anionic starch with a cationic polymer, for examplepolyvinylamine, at 80 to 220° C. in an aqueous medium. The aqueoussolutions thus obtainable are used as dry and wet strength enhancers andalso as drainage and retention aids for paper, paperboard and cardboard.

WO-A-98/45536 relates to a process for preparing paper, paperboard andcardboard of high dry strength by adding starches which are digested inan aqueous medium in the presence of cationic polymers. The paper stockis additionally admixed with a cationic polymer as retention aid forstarch. The higher retention reduces the COD of the machine wastewater.

Yet, with all the processes mentioned above, where a cationic modifiedstarch dry strength enhancer is added to paper stock, the sheet-formingstep is accompanied by an unwelcome reduction in the drainage rate ofthe paper stock, which is particularly pronounced when comparativelyhigh starch levels are used.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide modified starchwhich, in the production of paper, paperboard and cardboard, combines anenhanced retention of starch in the paper with a drainage rate superiorto that of existing cationic modified starches.

We have found that this object is achieved by a process for modifyingstarch by heating starch with at least one cationic polymer in anaqueous medium to temperatures above the gelatinization temperature ofsaid starch, which comprises effecting said modifying of said starch inthe presence of

(a) a polymeric cationizer selected from the group consisting of thepolymers containing vinylamine units and having molar masses of up to 1million, the polyethyleneimines, the polydiallyldimethylammoniumchlorides, the condensates of dimethylamine with epichlorohydrin ordichloroalkanes, the condensates of dichloroethane and ammonia, and themixtures thereof, and

(b) polymeric papermaking drainage aids selected from the groupconsisting of the water-soluble crosslinked polyamidoamines with orwithout an ethyleneimine graft, the nonionic or cationic polymerscontaining acrylamide and/or methacrylamide units and having molarmasses of more than 1 million, the polymers containing vinylamine unitsand having molar masses of more than 1 million, and the mixturesthereof.

The present invention also provides the reaction products of starch withcationic polymers obtainable by the process described above.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

All starches can be modified according to the invention, for examplenative starches, oxidized native starches, starch ethers, starch esters,oxidized starch ethers, oxidized starch esters, cationic modifiedstarches or amphoteric starches.

The native starches used may be starches from the group consisting ofmaize starch, potato starch, wheat starch, rice starch, tapioca starch,sago starch, sorghum starch, casaba starch, pea starch, or mixturesthereof. It is also possible to use starches which have an amylopectincontent of at least 80% by weight. Such starches are obtainable forexample by fractionation of customary native starches or by cultivationof such plants as produce virtually pure amylopectin starch, asdescribed in Günther Tegge, Stärke und Stärkederivate, Hamburg,Behr's-Verlag 1984, pages 157-160. Starches having an amylopectincontent of at least 80% by weight are commercially available. They aregenerally known as waxy maize starch, waxy potato starch or waxy wheatstarch.

As well as native starches, it is also possible to use hydrolytically orenzymatically degraded starches, for example dextrins, such as white oryellow dextrins and multidextrins, or oxidized starches, for exampledialdehyde starch. It is also possible to use chemically modifiedstarches, for example starches esterified with inorganic or organicacids, especially phosphated and acetylated starches, and also starchesetherified with organic halogen compounds, epoxides or sulfates.Starches and processes for their degradation and their chemicalmodification are described in Ullmann's Encyclopedia of IndustrialChemistry, 5^(th) edition, vol. A25, pages 2 ff; which is herebyincorporated herein by reference.

The oxidation of starches is described in Ullmann's Encyclopedia ofIndustrial Chemistry, Sixth edition, 1998. Such oxidized starches can beprepared by various reactions. The oxidation converts OH or aldehydefunctions, for example, into carboxylic acid functions. This oxidationmay also be accompanied by C₂-C₃ bond breakage. Oxidized starches may beprepared using any of the native starches mentioned.

The anionic modification of starch is described in Günther Tegge, Stärkeund Stärkederivate, Hamburg, Behr's-Verlag, 1984, pages 179 to 185. Ingeneral, anionic modification is a reaction of starch withα-halocarboxylic acids, preferably chloroacetic acid, or salts thereofor an esterification of starch with inorganic or organic acids asanionic modifier, preferably with dibasic or higher acids. Examples ofacids useful as anionic modifiers are orthophosphoric acid,metaphosphoric acid, pyrophosphoric acid, polyphosphoric acids,phosphorous acid, sulfuric acid, pyrosulfuric acid, thiosulfuric acid,orthosilica, metasilica, pyrosilica, polysilicas, orthoboric acid,metaboric acid, polyboric acids, O-esters of dithiocarbonic acid(xanthic acids), oxalic acid, malonic acid, succinic acid, glutaricacid, adipic acid, phthalic acid, terephthalic acid, malic acid, citricacid, etc. and salts thereof. The degree of esterification of theresultant anionic modified starches is generally about 0.01-100 mol %,preferably 0.1-50 mol %, especially 0.2-20 mol %, based on the freehydroxyl groups in the starch modified. Polybasic acids may also bepolyesterified. The process of the invention is preferably carried outusing oxidized starches.

Particular preference is given to the use of dialdehyde starches,prepared by oxidation of native starch, and to anionic starches. Theanionic starches preferably have carboxyl, phosphate or sulfate groupsor respectively the alkali metal or ammonium salts thereof. Particularlypreferably the anionic starch is carboxyl- and/or carboxylato-containingstarch from potatoes, maize, wheat or tapioca. Starches whose use isparticularly preferred are native potato starch, oxidized potato starchand oxidized maize starch. The oxidized starches contain carboxyl orcarboxylate groups as anionic groups. The degree of substitution of theanionic starches is within the range from 0.0001 to 0.3, for example.

According to the invention, at least one starch or a mixture of aplurality of starches is heated in an aqueous medium in the presence of

(a) at least one polymeric cationizer, and

(b) at least one polymeric drainage aid for paper.

Examples of suitable polymeric cationizers are polymers containingvinylamine units and having molar masses of up to 1 million. Polymers ofthis kind are prepared according to known processes by polymerizingN-vinylcarboxamides of the formula

where R, R¹=H or C₁-C₆-alkyl, alone or in the presence of other,copolymerizable monomers and hydrolyzing the resulting polymers withacids or bases to detach the group

and form units of the formula

where R is as defined for the formula (I).

Examples of suitable monomers of the formula (I) are N-vinylformamide,N-vinyl-N-methylformamide, N-vinyl-N-ethylformamide,N-vinyl-N-propylformamide, N-vinyl-N-isopropylformamide,N-vinyl-N-butylformamide, N-vinyl-N-sec-butylformamide,N-vinyl-N-tert-butylformamide, N-vinyl-N-pentylformamide,N-vinylacetamide, N-vinyl-N-ethylacetamide andN-vinyl-N-methylpropionamide. Polymers containing polymerized units ofthe formula (III) are preferably prepared using N-vinylformamide.

The hydrolyzed polymers which contain units of the formula (III) havefor example K values of from 15 to 150, preferably from 30 to 150,determined by the method of H. Fikentscher in aqueous solution at pH 7,25° C. and a polymer concentration of 0.5% by weight. Copolymers of themonomers (I) contain for example

1) from 99 to 1 mol % of N-vinylcarboxamides of the formula (I), and

2) from 1 to 99 mol % of other, copolymerizable monoethylenicallyunsaturated monomers,

for example vinyl esters of saturated carboxylic acids having from 1 to6 carbon atoms, e.g., vinyl formate, vinyl acetate, vinyl propionate andvinyl butyrate. It is also possible to use unsaturated C₃-C₆-carboxylicacids, e.g., acrylic acid, methacrylic acid, maleic acid, crotonic acid,itaconic acid and vinylacetic acid, and also their alkali metal andalkaline earth metal salts, esters, amides and nitriles, for examplemethyl acrylate, methyl methacrylate, ethyl acrylate and ethylmethacrylate, or glycol or polyglycol esters of ethylenicallyunsaturated carboxylic acids where in each case only one OH group in theglycols and polyglycols has been esterified, e.g., hydroxyethylacrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate,hydroxybutyl acrylate, hydroxypropyl methacrylate, hydroxybutylmethacrylate, and also the acrylic monoesters of polyalkylene glycolshaving a molecular weight of from 1500 to 10,000. Also suitable are theesters of ethylenically unsaturated carboxylic acids with aminoalcohols,e.g., dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate,diethylaminoethyl acrylate, diethylaminoethyl methacrylate,dimethylaminopropyl acrylate, dimethylaminopropyl methacrylate,diethylaminopropyl acrylate, diethylaminopropyl methacrylate,dimethylaminobutyl acrylate and diethylaminobutyl acrylate. The basicacrylates are used in the form of the free bases, the salts with mineralacids, for example hydrochloric acid, sulfuric acid and nitric acid, thesalts with organic acids such as formic acid or benzenesulfonic acid, orin quaternized form. Examples of suitable quaternizing agents aredimethyl sulfate, diethyl sulfate, methyl chloride, ethyl chloride andbenzyl chloride.

Useful comonomers 2) further include unsaturated amides such as, forexample, acrylamide, methacrylamide and also N-alkylmonoamides andN-alkyldiamides containing alkyl radicals of from 1 to 6 carbon atoms,e.g., N-methylacrylamide, N,N-dimethylacrylamide,N-methylmethacrylamide, N-ethylacrylamide, N-propylacrylamide andtert-butylacrylamide and also basic (meth)acrylamides, e.g.,dimethylaminoethylacrylamide, dimethylaminoethylmethacrylamide,diethylaminoethylacrylamide, diethylaminoethylmethacrylamide,dimethylaminopropylacrylamide, diethylaminopropylacrylamide,dimethylaminopropylmethacrylamide and diethylaminopropylmethacrylamide.

Suitable comonomers further include N-vinylpyrrolidone,N-vinylcaprolactam, acrylonitrile, methacrylonitrile, N-vinylimidazoleand also substituted N-vinylimidazoles, e.g., N-vinyl-2-methylimidazole,N-vinyl-4-methylimidazole, N-vinyl-5-methylimidazole,N-vinyl-2-ethylimidazole, and N-vinylimidazolines, e.g.,vinylimidazoline, N-vinyl-2-methylimidazoline, andN-vinyl-2-ethylimidazoline. N-Vinylimidazoles and N-vinylimidazolinesare used not only in the form of the free bases but also afterneutralization with mineral acids or organic acids or in quaternizedform, preferred quaternizing agents being dimethyl sulfate, diethylsulfate, methyl chloride and benzyl chloride.

Useful comonomers 2) further include sulfo-containing monomers such asvinylsulfonic acid, allylsulfonic acid, methallylsulfonic acid,styrenesulfonic acid and 3-sulfopropyl acrylate.

The copolymers further include terpolymers and such polymers asadditionally contain at least one further monomer in polymerized form.

Preferred cationic polymers containing vinylamine units are hydrolyzedcopolymers of

1) N-vinylformamide, and

2) vinyl formate, vinyl acetate, vinyl propionate, acrylonitrile andN-vinylpyrrolidone and also hydrolyzed homopolymers of N-vinylformamidehaving a degree of hydrolysis within the range from 2 to 100, preferablyfrom 30 to 95, mol %.

Copolymers containing vinyl esters in copolymerized form may behydrolyzed, depending on the hydrolysis conditions, not only at theN-vinylformamide units but also at the ester groups to form vinylalcohol units. Copolymerized acrylonitrile is likewise chemicallymodified by the hydrolysis, forming amide, amidine and/or carboxylgroups, for example. The hydrolyzed poly-N-vinylformamides mayoptionally contain up to 20 mol % of amidine structures formed byreaction of formic acid with two adjacent amino groups in thepolyvinylamine or by reaction of a formamide group with an adjacentamine group. The degree of hydrolysis of the N-vinylformamide units inthe homo- and copolymers is preferably within the range from 75 to 100%.The molar mass of the polymers which contain vinylamine units and whichare used as cationizers for starch is up to 1 million and is preferablywithin the range from 10,000 to 500,000 (determined by lightscattering).

Useful cationic polymers further include compounds containingpolymerized ethyleneimine units. These compounds are preferablypolyethyleneimines obtainable by polymerizing ethyleneimine in thepresence of acidic catalysts such as ammonium hydrogensulfate,hydrochloric acid or chlorinated hydrocarbons such as methyl chloride,ethylene dichloride, butyl chloride, carbon tetrachloride or chloroform.Such polyethylenimines have a viscosity of from 500 to 33,000,preferably from 1000 to 31,000 mPaVs (measured according to Brookfieldat 20° C. and 20 rpm), in 50% strength by weight aqueous solution, forexample.

Useful cationizers for starch further includepoly(diallyldimethylammonium chloride)s. Polymers of this kind areknown. Diallyldimethylammonium chloride polymers contemplated areprimarily homopolymers and also copolymers with acrylamide and/ormethacrylamide. Any copolymerization may be carried out in any desiredmonomer ratio. The molar mass of the homo- or copolymers ofdiallyldimethylammonium chloride is not less than 50,000, preferablyfrom 50,000 to 500,000 (determined by light scattering).

Useful cationizers for starch further include condensates ofdimethylamine with epichlorohydrin or dichloroalkanes such as1,2-dichloroethane, 1,2-dichloropropane, 1,3-dichloropropane,1,4-dichlorobutane and/or 1,6-dichlorohexane, or condensates of1,2-dichloroethane and ammonia. The condensates mentioned are soluble inwater and have molar masses of up to 1 million, preferably from 50,000to 500,000, for example.

Starch modification requires not only the above-described polymericcationizers but also the polymeric papermaking drainage aids under group(b).

Examples of polymeric drainage aids are polyamidoamines with or withoutan ethyleneimine graft. Such polymers are described in U.S. Pat. No.4,144,123, for example. Polyamidoamines are prepared for example byreacting dicarboxylic acids of from 4 to 10 carbon atoms withpolyalkylenepolyamines which preferably contain from 3 to 10 basicnitrogen atoms in the molecule. Examples of suitable dicarboxylic acidsare succinic acid, maleic acid, adipic acid, glutaric acid, subericacid, sebacic acid and terephthalic acid. It is also possible to usemixtures of adipic acid and glutaric acid or maleic acid and adipicacid. Polyamidoamines are preferably prepared using adipic acid.

Examples of suitable polyalkylenepolyamines which are condensed with thedicarboxylic acids are diethylenetriamine, triethylenetetramine,dipropylenetriamine, tripropylenetetramine, dihexamethylenetriamine,aminopropylethylenediamine and bisaminopropylethylenediamine.Polyalkylenepolyamines can also be used in the form of mixtures toprepare polyamidoamines. Polyamidoamines are preferably prepared bycondensation of dicarboxylic acids and polyamines in the absence of asolvent. However, the condensation can also be carried out in inertsolvents, if necessary. The condensation of dicarboxylic acids withpolyalkylenepolyamines is customarily carried out at from 100 to 220°C., for example, and the water formed in the course of the reaction isdistilled out of the reaction mixture. The condensation may also becarried out in the additional presence of lactones or lactams ofcarboxylic acids having from 4 to 8 carbon atoms, for example in thepresence of caprolactam. From 0.8 to 1.4 mol of a polyalkylenepolyamineis used per mole of a dicarboxylic acid, for example. Polyamidoaminesthus obtainable have primary and secondary NH groups, plus tertiarynitrogen atoms, in certain circumstances, and are soluble in water.

The above-described polyamidoamines can be modified by grafting withethyleneimine by, for example, allowing ethyleneimine to act on thepolyamidoamines in the presence of acids (e.g., sulfuric acid orphosphoric acid) or in the presence of Lewis acids (e.g., borontrifluoride etherates). For example, per basic nitrogen group in thepolyamidoamine, from 1 to 50, preferably from 2 to 25, ethyleneimineunits can be grafted on; i.e., about 10-500 parts by weight ofethyleneimine are used per 100 parts by weight of a polyamidoamine, forexample.

Useful polymeric drainage aids further include reaction products whichare preparable by grafting polyamidoamines with ethyleneimine andsubsequent reaction with crosslinkers containing at least two functionalgroups. Products of this kind are identified in the hereinabovepreviously cited U.S. Pat. No. 4,144,123, for example, as usefulretention, flocculation and drainage aids in papermaking.

Useful crosslinkers containing at least two functional groups includenot only the polyalkylene glycol bischlorohydrin ethers described inU.S. Pat. No. 4,144,123, but also α,ω-dichloropolyalkylene glycols knownas crosslinkers from EP-B-0 025 515, for example.

Useful crosslinkers for the reaction with optionallyethyleneimine-grafted polyamidoamines include for example α,ω- orvicinal dichloroalkanes, for example 1,2-dichloroethane,1,2-dichloropropane, 1,3-dichloropropane, 1,4-dichlorobutane and1,6-dichlorohexane. Examples of further crosslinkers are the reactionproducts of at least trihydric alcohols with epichlorohydrin to formreaction products which have at least two chlorohydrin units. Examplesof polyhydric alcohols used are glycerol, ethoxylated or propoxylatedglycerols, polyglycerols containing from 2 to 15 glycerol units in themolecule and also optionally ethoxylated and/or propoxylatedpolyglycerols. Crosslinkers of this kind are known from DE-A-2 916 356,for example. It is also possible to use crosslinkers which containblocked isocyanate groups, e.g., trimethylhexamethylene diisocyanateblocked with 2,2,3,6-tetramethylpiperidin-4-one. Such crosslinkers areknown, cf. for example DE-A-4 028 285, and also crosslinkers whichcontain aziridine units and are based on polyethers or substitutedhydrocarbons, e.g., 1,6-bis-N-aziridinohexane, cf. U.S. Pat. No.3,977,923. It will be appreciated that it is also possible to usemixtures of two or more crosslinkers to increase the molecular weight.

Preference is given to using halogen-free crosslinkers, for example di-or polyepoxides, ethylene carbonate and/or propylene carbonate. Thereaction with the crosslinkers is conducted so as to obtainwater-soluble products having a viscosity of from 50 to 5000, preferablyfrom 100 to 2000, mPas at 20° C. in the form of a 10% strength by weightaqueous solution, for example.

Useful polymeric drainage aids further include nonionic or cationicpolymers containing acrylamide and/or methacrylamide units and havingmolar masses of more than 1 million. Such high molecular weight polymersare customarily used as drainage and retention aids in papermaking.Preferred examples of polymers of this kind are high molecular weightpolyacrylamides, high molecular weight polymethacrylamides and also thecopolymers of acrylamide and methacrylamide with copolymerizablemonomers. Examples of suitable drainage aids are copolymers of 60-85% byweight of acrylamide and/or methacrylamide and 15-40% by weight ofN-vinylimidazoline or N-vinyl-2-methylimidazoline. The copolymers mayfurther be modified by incorporation of polymerized units from othermonomers such as styrene, N-vinylformamide, vinyl formate, vinylacetate, vinyl propionate, C₁-C₄-alkyl vinyl ethers, N-vinylpyridine,N-vinylpyrrolidone, N-vinylimidazole, ethylenically unsaturatedC₃-C₅-carboxylic acids and also their esters, amides and nitriles,sodium vinylsulfonate, 2-acrylamidomethylpropanesulfonic acid, vinylchloride and vinylidene chloride in amounts of up to 25% by weight.Monomers which contain acid groups are preferably used in the form ofthe alkali metal or ammonium salts in the copolymerization. Examples ofuseful drainage aids are copolymers containing

1) from 70 to 97% by weight of acrylamide and/or methacrylamide,

2) from 2 to 20% by weight of N-vinylimidazoline orN-vinyl-2-methylimidazoline, salts of these monomers and/oralkylator-quaternized N-vinylimidazoline or N-vinyl-2-methylimidazoline,and

3) from 1 to 10% by weight of N-vinylimidazole

in copolymerized form. These copolymers are prepared by free radicalcopolymerization of said monomers 1), 2) and 3) according to knownpolymerization processes. They have molar masses of >1 million,preferably of from 1.2 to 30 million.

Useful high molecular weight cationic polymers further includecopolymers of 1-99 mol %, preferably 30-70 mol %, of acrylamide and/ormethacrylamide and 99-1 mol %, preferably 70-30 mol %, ofdialkylaminoalkyl acrylates and/or methacrylates, for example copolymersof acrylamide and N,N-dimethylaminoethyl acrylate orN,N-diethylaminoethyl acrylate. Basic acrylates are preferably presentin acid-neutralized or in quaternized form. Quaternization may beeffected for example with methyl chloride or with dimethyl sulfate.These copolymers have a charge density of at least 4 meq/g ofpolyelectrolyte at a pH of 4.5, for example.

Useful drainage aids also include high molecular weight copolymers offrom 1 to 99 mol %, preferably of from 30 to 70 mol %, of acrylamideand/or methacrylamide and of from 99 to 1 mol %, preferably of from 70to 30 mol %, of dialkylaminoalkylacrylamide and/ordialkylaminoalkylmethacrylamide. Basic acrylamides and methacrylamidesare preferably likewise present in acid-neutralized or in quaternizedform. Specific examples are N-trimethylammoniumethylacrylamide chloride,N-trimethylammoniumethylmethacrylamide chloride,trimethylammoniumethylacrylamide methosulfate,trimethylammoniumethylmethacrylamide methosulfate,N-ethyldimethylammoniumethylacrylamide ethosulfate,N-ethyldimethylammoniumethylmethacrylamide ethosulfate,trimethylammoniumpropylacrylamide chloride,trimethylammoniumpropylmethacrylamide chloride,trimethylammoniumpropylacrylamide methosulfate,trimethylammoniumpropylmethacrylamide methosulfate andN-ethyldimethylammoniumpropylacrylamide ethosulfate.Trimethylammoniumpropylmethacrylamide chloride is preferred. The molarmass of these polymers is likewise above 1 million.

Useful high molecular weight polymeric drainage aids further includepolymers containing vinylamine units and having molar masses of morethan 1 million. These polymers may have the same percentage compositionas the vinylamine polymer cationizers mentioned under (a). Thedifference is the higher molecular weight of polymers (b). Examples ofpreferred high molecular weight polymers containing vinylamine units arehigh molecular weight hydrolyzed poly-N-vinylformamides having a degreeof hydrolysis of from 20 to 100%, preferably of from 75 to 100%.Preferred vinylamine polymers further include hydrolyzed copolymers ofN-vinylformamide and vinyl formate, vinyl acetate, vinyl propionate,acrylonitrile or N-vinylpyrrolidone. The degree of hydrolysis of thesepolymers is for example within the range from 2 to 100 mol %, preferablywithin the range from 30 to 95 mol %. The molar mass of this group ofpolymers with [sic] likewise above 1 million, for example within therange from 1.2 to 30 million, preferably within the range from 1.5million to 15 million.

To modify a starch or a mixture of starches according to the presentinvention, for example, an aqueous suspension of at least one starchvariety is heated with a cationic polymeric cationizer of group (a) anda polymeric papermaking drainage aid of group (b) at temperatures abovethe gelatinization temperature of starch. The initial result of heatingaqueous starch suspensions in the presence of polymers of groups (a) and(b) is the digestion of the starch, i.e., the conversion of the solidstarch grains into a water-soluble form, with disappearance ofsuperstructures (helix formation, intramolecular hydrogen bonds, etc.),without the amylose and/or amylopectin units which make up the starchbeing degraded to oligosaccharides or glucose. The aqueous starchsuspensions are preferably heated in the presence of at least onepolymeric cationizer (a) and at least one polymeric papermaking drainageaid (b) to temperatures above the gelatinization temperature of starch.These temperatures are for example within the range from 80 to 180° C.,preferably within the range from 105 to 170° C. When temperatures offrom 105 to 170° C. are employed, the starch is heated undersuperatmospheric pressure. In the course of being heated in aqueoussuspension, the starch is digested to an extent of not less than 90%,preferably more than 90%, by weight and modified by the two differenttypes of polymers. Preferably said conversion of the starch is effectedwith the polymers a) and b) under superatmospheric pressure in a jetcooker at from 115 to 150° C. in the course of from 0.01 sec to 30minutes. In the course of the reaction in a jet cooker, the reactionmixture is subjected to shearing. However, if the reaction is carriedout in a stirred autoclave, the reaction mixture is stirred at from 100to 2000 rpm, preferably at from 200 to 1000 rpm, for example. Thereaction can be carried out in virtually any apparatus which areemployed in industry for starch digestion, and is preferably carried outin pressure tight stirred tanks or preferably in jet cookers. When thereaction is carried out in a stirred autoclave, the residence time ofthe reaction mixture at the reaction temperatures contemplated is forexample within the range from 2 min to 2 hours, preferably within therange from 15 to 60 min.

Starch may also be modified, however, by first heating an aqueoussuspension of the starch to temperatures above the gelatinizationtemperature of starch to digest the starch, then—likewise attemperatures above the gelatinization temperature of starch—adding thepolymeric cationizers (a) allowing them to react, and thereafter addingthe polymeric papermaking drainage aid (b), allowing the resultantreaction mixture to react for from 1 sec to 30 min, for example, attemperatures above the gelatinization temperature of starch, andthereafter cooling down the reaction mixture. However, it is alsopossible to treat a previously digested starch in aqueous solutioninitially with a polymeric drainage aid (b) at temperatures above thegelatinization temperature of starch (for example from 1 sec to 1 h) andthen adding a polymeric cationizer (a) and allowing the reaction mixtureto react at temperatures above the gelatinization temperature of starch,again for from 1 sec to 1 h. In the preferred procedure, however, anaqueous suspension of starch is simultaneously digested and modified byheating it in a jet cooker at from 115 to 150° C. for from 0.01 sec to30 min in the presence of polymers (a) and (b). Said polymers (a) and(b) may be added to the starch suspension in the form of a mixture orseparately. It is similarly possible to meter polymers (a) and (b) insuccession into the reaction zone of the jet cooker.

The concentration of starch in the aqueous starch suspension is forexample within the range from 2 to 15% by weight, preferably within therange from 6 to 10% by weight. Per 100 parts by weight of the starchused as starch to be modified, the amount of polymeric cationizer (a)used is for example within the range from 0.1 to 10 parts by weight,preferably within the range from 0.5 to 3 parts by weight, and that ofthe polymeric drainage aid (b) within the range from 0.01 to 2 parts byweight, preferably within the range from 0.1 to 0.9 part by weight. Theaqueous modified starch solutions obtained have a viscosity of from 10to 500, preferably of from 50 to 200, mPas, measured in a Brookfieldviscometer at 20 rpm and 40° C., at a solids concentration of 2% byweight, for example. The pH of the reaction mixtures is for examplewithin the range from 2.0 to 9.0, preferably within the range from 2.5to 8.

The modified starches thus obtainable are used as paper, paperboard andcardboard dry strength enhancers. They are added to the paper stock inamounts of from 0.5 to 8.0%, preferably of from 1.0 to 6.0%, by weight,based on dry paper stock, for example. The otherwise customary processchemicals for papermaking may be additionally employed, for example asizing agent or else a retention aid in addition to the dry strengthenhancers. Examples of useful retention aids are high molecular weightpolyacrylamides or high molecular weight poly-N-vinylformamides having adegree of hydrolysis of from 25 to 100%. The molar masses of retentionaids are customarily above 1 million, for example within the range from1.5 to 30 million.

Useful retention aids for the starch modified according to the inventionfurther include so-called microparticulate systems under which a highmolecular weight cationic synthetic polymer is added to the paper stock,the macroflocs are divided by shearing the paper stock and thenbentonite is added. This process is known from EP-A-0 335 575, forexample. For such a microparticulate system it is possible, for exampleto use, as the high molecular weight cationic polymers having molarmasses of >1 million, a mixture of a vinylamine polymer, for examplepolyvinylamine, and a cationic polyacrylamide, for example a copolymerof acrylamide and dimethylaminoethyl acrylate methochloride and to addbentonite after the shearing step. Further preferred combinations ofcationic polymers useful as retention aids for starches modifiedaccording to the invention are mixtures of vinylamine polymers havingmolar masses of >1 million and ethyleneimine-grafted crosslinkedpolyamidoamines and also mixtures of said high molecular weightvinylamine polymers with polyacrylamides having molar masses of from 2million to 15 million.

Unless otherwise stated, percentages in the Examples are by weight. TheK values were determined according to H. Fikentscher, Cellulose-Chemie13 (1932), 58-64, 71-74, in aqueous solution at a polymer concentrationof 0.5% by weight and at 25° C. The molar masses of the polymers weredetermined by light scattering. The viscosities were measured in aBrookfield viscometer at 20 rpm.

EXAMPLES

The following materials were used:

Cationizer I

Hydrolyzed poly-N-vinylformamide which contained 95 mol % of vinylamineunits and 5 mol % of vinylformamide units and had a molar mass of120,000 g/mol and a viscosity of 600 mPas in 7.8% strength by weightaqueous solution at 20° C. and pH 7.0. The vinylamine polymer wasprepared by hydrolyzing a K 70 N-vinylformamide polymer using aqueoussodium hydroxide solution.

Drainage Aid I

Commercially available ethyleneimine-grafted water-solublepolyamidoamine formed from adipic acid and a triamine and crosslinkedwith a bischlorohydrin ether (Polymin® SK).

Starch I

Oxidized maize starch having carboxyl groups as anionic groups. Thedegree of substitution of this starch is 0.8 mmol/mol (=0.8×10⁻³).

Inventive Example 1

A 10% strength aqueous suspension of starch I was produced, 1.5%, basedon starch, of cationizer I and 0.4% of drainage aid I were added, andthe mixture was fed into a jet cooker. It was maintained therein at 130°C. for 1 min and then discharged, affording an aqueous solution of amodified anionic maize starch.

Comparative Example 1

A 10% strength aqueous slurry of starch I was admixed with 1.5%, basedon starch I, of cationizer I. The mixture was then cooked in a jetcooker at 130° C. for 1 min, affording an aqueous solution of a starch Imodified by cationizer I.

Inventive Example 2

An experimental papermachine was used to produce paper having a basisweight of 120 g/m² at a production speed of 50 m/min. The fiber used wasbeaten commercially available liner having a concentration of 12 g/l anda pH of 7.0. This stock was admixed, in each case based on dry paperstock, with 2% of the modified starch prepared according to inventiveexample 1 and as retention aid with a combination of 0.04% of drainageaid I and 0.001% of a commercially available polyacrylamide having amolar mass of 5 million. After the paper web formed had passed throughthe press end of the paper machine, samples were taken to determine themoisture content of the paper. The dry content of the paper was 66.4%.

Also determined were the dry bursting pressure (according to DIN ISO2758), the flat crush resistance (CMT value according to DIN EN23035=ISO 3035) and the dry breaking length (according to DIN ISO 1924).The dry bursting pressure was found to be 223 Pa, the CMT value 227 Nand the dry breaking length 5545 m.

Inventive Example 3

Inventive example 2 was repeated with the one difference that 6% of thestarch modified according to inventive example 1 was added to the paperstock, based on solids. After passing through the press end of the papermachine, the dry content of the paper was 62.7%. The finished paper hada dry bursting pressure of 293 Pa, a CMT value of 268 N and a drybreaking length of 6371 m.

Comparative Example 2

Inventive example 2 was repeated with the one difference that themodified starch used was replaced by the modified starch obtainedaccording to comparative example 1. After passing through the press end,the paper had a dry content of 51.8%. The finished paper had a drybursting pressure of 205 mPa, a CMT value of 199 N and a dry breakinglength of 5261 m.

Comparative Example 3

Inventive example 3 is repeated with the one difference that themodified starch obtained according to comparative example 1 is used. Thepapermaking process had to be discontinued at the predeterminedpapermachine speed, since the fiber did not drain sufficiently rapidly.

As inventive examples 2 and 3 show, a higher use level of starch ispossible compared with comparative examples 2 and 3, and the inventionprovides better drainage of the paper stock (higher dry content of thepaper downstream of press end of papermachine) and a high level for thestrength values of the paper.

We claim:
 1. A process for modifying starch, comprising: heating starchwith at least one cationic polymer in an aqueous medium to temperaturesabove the gelatinization temperature of said starch, wherein the starchis selected from the group consisting of the native starches, theoxidized native starches, the starch ethers, the starch esters, theoxidized starch ethers, the oxidized starch esters, the cationicmodified starches and the amphoteric starches and effecting saidmodifying of said starch in the presence of a combination of (a) apolymeric cationizer selected from the group consisting of polymerscontaining vinylamine units and having molecular weights Mw of up to 1million, polyethyleneimines, polydiallyldimethylammonium chlorides,condensates of dimethylamine with epichlorohydrin or dichloroalkanes,condensates of dichloroethane and ammonia, and mixtures thereof; and (b)a polymeric papermaking drainage aid selected from the group consistingof a water-soluble crosslinked polyamidoamine with or without anethyleneimine graft, a polymer containing acrylamide and/ormethacrylamide units and having a molecular weight Mw of more than 1million, a polymer containing vinylamine units and having a molecularweight Mw of more than 1 million, and mixtures thereof.
 2. The processas claimed in claim 1, wherein (a) said polymeric cationizer is selectedfrom the group consisting of polyethyleneimines, polymers containingvinylamine units and having molecular weights Mw of up to 1 million, and(b) said polymeric drainage aid is selected from the group consisting ofa water-soluble, ethyleneimine-grafted crosslinked polyamidoamine, apolymer containing vinylamine units and having a molecular weight Mwranging from 1.2 to 30 million, and a cationic polyacrylamide ornonionic polyacrylamide each having a molecular weight Mw of not lessthan 1.5 million.
 3. The process as claimed in claim 1, wherein saidstarch is a native starch.
 4. The process as claimed in claim 1, whereinsaid starch is selected from the group consisting of the starch ethers,the starch esters, the oxidized native starch, the oxidized starchethers and the oxidized starch esters.
 5. The process as claimed inclaim 1, wherein said starch, prior to modification is an anionicstarch.
 6. The process as claimed in claim 1, wherein said starch is ananionic starch that contains carboxyl and/or carboxylato, phosphate orsulfate groups or the respective alkali metal or ammonium salts thereof.7. The process as claimed in claim 6, wherein said anionic starch thatis a carboxyl- and/or carboxylato-containing starch is obtained frompotatoes, maize, wheat or tapioca.
 8. The process as claimed in claim 1,wherein said starch is heated in said aqueous medium to 115-170° C.under superatmospheric pressure.
 9. The process as claimed in claim 1,wherein said heating of said starch is effected with at least onepolymeric cationizer and at least one polymeric drainage aid in a jetcooker at a temperature ranging from 120 to 150° C. in the course offrom 0.01 sec to 30 minutes.
 10. The process as claimed in claim 1,wherein, based on 100 parts by weight of starch, from 0.1 to 10 parts byweight of at least one polymeric cationizer (a) is combined with from0.01 to 2 parts by weight of at least one drainage aid (b) for themodification of said starch.
 11. A reaction product of starch with acationic polymer prepared by the process of claim
 1. 12. A paper,paperboard or cardboard product, comprising: a paper, paperboard orcardboard modified with from 0.5 to 8.0% by weight of the reactionproduct of claim
 11. 13. A process for producing paper, paperboard orcardboard, comprising: adding a dry strength enhancer prepared by theprocess of claim 1 to a paper stock; and draining said treated paperstock onto a sieve which results in the formation of a sheet on thesieve, thereby forming a paper, paperboard or cardboard of improved drystrength.
 14. The process as claimed in claim 1, wherein said drainageaid is a copolymer prepared by reacting: i) from 70 to 97% by weight ofacrylamide and/or methacrylamide, ii) from 2 to 20% by weight ofN-vinylimidazoline or N-vinyl-2-methylimidazoline, salts of thesemonomers and/or alkylator-quaternized N-vinylimidazoline orN-vinyl-2-methylimidazoline, and iii) from 1 to 10% by weight ofN-vinylimidazole.
 15. The process as claimed in claim 1, wherein saiddrainage aid is a copolymer prepared by reacting: i) from 1 to 99 mol %of (meth)acrylamide with ii) from 99 to 1 mol % of adialkylaminoalkyl(meth)acrylamide.
 16. The process as claimed in claim1, wherein the pH of the starch modification reactions ranges from 2.0to 9.0.
 17. The process as claimed in claim 1, wherein saidpolyamidoamine drainage aid has from 1 to 50 ethyleneinine units graftedthereon per basic nitrogen atom of the polyamidoamine.